For time travel stories, my impression is that despite the different names, they all involve inconsistent causal loops, unless the multiverse is introduced. The difficult part of time travel paradoxes is to think out of all of the possible ramifications due to the circular nature of events.

Spoilers for: 12 Monkey (movie), STNG: All Good Things, Interstellar, Back To The Future

In the Back to the Future movie, the concept is easy to understand that if McFly does not get his parents together, he will cease to exist, by virtue of a fading photograph, which as I recall ,a photo was used in both the 1st and 3rd movies. However what the movie does not address is all of the peoples' lives he has effected by interacting with them, other than the benefit to his own family. Maybe he inspired the "soda jerk" to become mayor. This is an altered timeline. Best not to think about all the changes that occured to Marty by virtue of the altered time line, and how he would mesh returning back to his now changed former life.

In 12 Monkeys, a character Cole is sent back in time to look for an organization called the The Army of The 12 Monkeys, an organization believed to be responsible for the outbreak of a deadly disease that wipes out most of humanity. However he arrives earlier than the target date, inquires, and inspires an inmate at a mental institution, which starts a chain of events. This would be a bootstrap paradox.

In Interstellar, a character from the future leaves clues in the past so the path forward can be established. Future Cooper leaves hints to his daughter in the book shelf, her ghosts, which helps himself move forward.

However my favorite paradox occurred in Star Trek Next Generation series final All Good Things where Capt Piccard finds himself traveling through time, jumping back and forth in his life. This is another test perpetrated by Q. In the past, he becomes aware of a temporal anomaly close to the Devron System in the Neutral Zone and discovers that by virtue of being a temporal anomaly, it is moving backwards in time, growing as it moves backwards. His retired self vowes to cajole his friends into transporting him to the Devron System to look for it at an earlier state, searching by means of deploying a reverse tachyon beam. The cause of the paradox is that he is the one who causes its creation by looking for it!

Jumangi- A favorite movie, no time travel, but creates a time line that is later erased when the game concludes. No paradox, simple to understand.

One of the earliest time travel stories I am aware of is The Sound of Thunder, a 1952 Ray Bradbury story which is the origin for the term "butterfly effect", where a time traveler steps on a butterfly and the future is changed.

Looper- the worst time travel movie I've seen if the goal is to all most understand why things happened the way they did. Nine Problems With Looper.

Rules of Time Travel@ Wikipedia.
1. Single fixed history, which is self-consistent and unchangeable. In this version, everything happens on a single timeline which does not contradict itself and cannot interact with anything potentially existing outside of it.
2. History is flexible and is subject to change (Plastic Time). Events can be altered, but there are different variations of this branch from time is easy to change (Back To The Future) to major changes are hard to achieve.
3. Alternate timelines. In this version of time travel, there are multiple coexisting alternate histories, so that when the traveler goes back in time, he/she ends up in a new timeline where historical events can differ from the timeline he/she came from, but his/her original timeline does not cease to exist (this means the grandfather paradox can be avoided since even if the time traveler's gran

Three types of Time Travel- Fixed Timeline, Dynamic Timeline, and Multiverse.
-Fixed Timeline- Even when parties travel back in time... the future they left cannot be changed. All events remain as fixed poinst in time. The actions of the traveler in the past have already become part of hisotry. This is known as the Novikov Self-Consistency Principle. For Example, say you travel back in time in order to kill Adolf Hitler as a baby in order to prvent WWII. You replace him with a orphaned baby, so that the family will not notice. You trave back to the Future, and the replaced baby grows up to becom Adolf Hitler Himself (The Terminator, Harry Potter 3, 12 Monkeys).
-Dynamic Timeline- Altered events in the past have definite impact on the present. For example: If you travel back in time and kill your Grandfather... you also prevent your own birth, and your eventual trip back in time, in turn, your Grandfather is never killed, and you are born again, only to go back in time and kill Your Grandfather anyway. A Paradox as seen in Back to the Future. Confusing? YES.
-Alternate Timelines- With an infinite number of parallel universes, traveling into the past causes a new divergent timeline from the first. Because of this, the traveler can do anything with impunity, and only the new timeline will be effected. For example, if you kill all your grandparents, nothing happens. There is no paradox, you have simply created a new timeline in which you will not exit, but the original timeline is unaffected. However, you cannot return to the original timeline. (But I assume you are still existing in the new reality you created, you just magically appeared there?)

The bootstrap paradox, or ontological paradox, is a paradox of time travel that refers to scenarios whereby items or information are passed from the future to the past, which in turn become the same items or information that are subsequently passed from the past to the future - this creates a circularity of cause-effect such that the items or information have no discernible origin. Thus, the paradox raises the ontological questions of where, when and by whom the items were created or the information derived.

You're a time-traveling assassin, and your target just happens to be your own grandfather. So you pop through the nearest wormhole and walk up to a spry 18-year-old version of your father's father. You raise your laser blaster, but just what happens when you pull the trigger?
Think about it. You haven't been born yet. Neither has your father. If you kill your own grandfather in the past, he'll never have a son. That son will never have you, and you'll never happen to take that job as a time-traveling assassin. You wouldn't exist to pull the trigger, thus negating the entire string of events. We call this an inconsistent causal loop.

In the Back to the Future movie, the concept is easy to understand that if McFly does not get his parents together, he will cease to exist, by virtue of a fading photograph, which as I recall ,a photo was used in both the 1st and 3rd movies. However what the movie does not address is all of the peoples' lives he has effected by interacting with them, other than the benefit to his own family. Maybe he inspired the "soda jerk" to become mayor.

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What I like about Back to the Future is the inconsistency. How come in one his changing the past threatens his existence? When Biff changes the past it creates a divergent timeline.

Going by the BtF 2 rules. When Marty prevented his parents relationship. He would have just created a divergent timeline. While he would still exist in his universe. When he went to the future it would have been in a universe which he did not exist.

Which is also the only possible outcome. Preventing his own conception would create a impossible paradox without a divergent timeline and universe. By preventing his conception he would not exist to have prevented it. Nor would the time machine have made it back.

Of course there is also the problem of the Time Machine only being able to go to the same position in the past or the future that it departed from. This would mean that any time travel would most likely result it appearing in outer space. As the universe is mostly empty space.

Since at any time the total speed of the Earth moving through the universe is between ~750,000 and ~1,900,000 MPH. If you could only go to the same place you departed from. Earth would be around 200 to 500 trillion miles away from you when you arrived about 30 years in the past or the future.

What I like about Back to the Future is the inconsistency. How come in one his changing the past threatens his existence? When Biff changes the past it creates a divergent timeline.

Going by the BtF 2 rules. When Marty prevented his parents relationship. He would have just created a divergent timeline. While he would still exist in his universe. When he went to the future it would have been in a universe which he did not exist.

Which is also the only possible outcome. Preventing his own conception would create a impossible paradox without a divergent timeline and universe. By preventing his conception he would not exist to have prevented it. Nor would the time machine have made it back.

Of course there is also the problem of the Time Machine only being able to go to the same position in the past or the future that it departed from. This would mean that any time travel would most likely result it appearing in outer space. As the universe is mostly empty space.

Since at any time the total speed of the Earth moving through the universe is between ~750,000 and ~1,900,000 MPH. If you could only go to the same place you departed from. Earth would be around 200 to 500 trillion miles away from you when you arrived about 30 years in the past or the future.

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I think most paradoxes require a suspension of disbelief. That's a great point about location, that I've not considered before. You'd just use your time/space movement calculator to figure out the new location along with the time. I'll need to watch the second one again as I don't remember the "alternate time line" aspect of the story. I'm most clear on the 1st and 3rd movies.

I was in Jr. High when this show came out and it was cutting edge for the time. My Mom did not want me watching it. My only complaint with this serie's visits to Earth, is that most times it looked like the studio back lot. The episode that left the biggest impression on my young impressionable mind was the episode where the guy's wife was really a morphing salt sucker who was murdering crew members.

Of course there is also the problem of the Time Machine only being able to go to the same position in the past or the future that it departed from. This would mean that any time travel would most likely result it appearing in outer space. As the universe is mostly empty space.

Since at any time the total speed of the Earth moving through the universe is between ~750,000 and ~1,900,000 MPH. If you could only go to the same place you departed from. Earth would be around 200 to 500 trillion miles away from you when you arrived about 30 years in the past or the future.

Edit: Correction that would be 200 to 400 billion miles.

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I loved the BTF films but yea. Anyway cool as the Delorean is, Time Lord technology has that beat-after all the TARDIS is a machine that can travel anywhere in "time and space", the doctor always describes it like that. Of course Doctor Who has it's own paradoxes.

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Evidently Steven King has a book our 11/23/63 that is being made into a movie (mini series I believe). This looks interesting where a man goes back in time to stop the Kennedy assassination. The book is highly rated and I may read it once I'm done with my current book.

Evidently Steven King has a book our 11/23/63 that is being made into a movie (mini series I believe). This looks interesting where a man goes back in time to stop the Kennedy assassination. The book is highly rated and I may read it once I'm done with my current book.

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I'll be curious to see King's take on time. Of interest, under the Rules of Time in the OP, History Is Flexible (plastic time) section, there are variations where on one end of the spectrum time is easy to change (Back to the Future) to the other end of the spectrum where insignificant events are easily altered, but significant events like an assination would be much more difficult to succeed.

The thing about Marty McFly was that if he had caused his parents not to get together, he would have never been born, but because he had split from the original time line, he did exist regardless. I think if he had failed, he would have returned to a world where he had never been born, similiar to It's A Wonderful Life when George Bailey is granted his wish, never to have been born... but he still existed.

Its interesting that the new link mentions a Young Adult series called Timeriders where one of the plots is to go back and prevent Kennedy's assassination. To me that would be a huge gamble playing with the Law of Unintended Consequences.

I liked the notion behind Thrillseekers (a naff pretty low budget execution but rather disturbing concept) where folks could travel back in time but strictly as observers of events - which spawns a sick underground tourist industry of folks travelling back to observe big historical disasters.

Course that leaves you with the problem of say couple of hundred folks all booking a ringside seat on titanic over the course of ten or twenty years - despite them all booking individually they'd all be attending the same past historical event.

So whether you're the first to book or the last to book, everyone else who ever signs up for the trip is going to be there too

I remember when I was a very small kid seeing the series 'Time Tunnel' (which at the time I thought absolutely spell binding) and which had come from the US.

I recall watching the first episode with my mother - which involved time travelling to the Titanic, the first time I had ever heard of this particular maritime disaster - and being stupefied by the fact that my mother - along with the pair of reluctant time travellers was adamant that this ship would actually sink.

The glaringly obvious problem in Back to the Future II was the fact that Biff made it back to the same 'future' when he returned in the delorean. That's not possible in Back to the Futures own terms of time travel. Hence, Marty and the Doc could not return to 1985 to find Biffs dynasty in Hill Valley.

But, it is an all time favourite for me. If you can't eject your brain for Back to the Future, I pity you.

I remember when I was a very small kid seeing the series 'Time Tunnel' (which at the time I thought absolutely spell binding) and which had come from the US.

I recall watching the first episode with my mother - which involved time travelling to the Titanic, the first time I had ever heard of this particular maritime disaster - and being stupefied by the fact that my mother - along with the pair of reluctant time travellers was adamant that this ship would actually sink.

And it did. I was awestruck. The things adults knew. Amazing, really.

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You'll be happy to hear that The Time Tunnel is available on hulu.com. In fact, I watched the first episode to which you refer recently. The time travel episodes on Star Trek TOS were also memorable, particularly the ones where the Enterprise travelled back to then present-day Earth. Assignment: Earth, in which Kirk and Spock first try to foil and then work with agent Gary Seven is my favorite. The part where Roberta Lincoln (played by a young Teri Garr) is dumbfounded by the typewriter that transcribes everything she says was great. (Of course, we now take that for granted, and it's only the 21st century.)

The most riveting time travel movie I've ever seen was "Primer," which took several viewings to begin to comprehend. It was shot for only $7k and is available to rent or buy on iTunes.

I recently saw Predesination with Ethan Hawke. It wasn't the best movie... it was quite odd to be honest. I don't want to give it away, but I saw the first twist regarding the Woman's relationship to her past lover. The twist at the end caught me completely off guard and I beat myself up a little for not considering it. Like I said, the movie wasn't great but the ending twist blew my mind.

You'll be happy to hear that The Time Tunnel is available on hulu.com. In fact, I watched the first episode to which you refer recently. The time travel episodes on Star Trek TOS were also memorable, particularly the ones where the Enterprise travelled back to then present-day Earth. Assignment: Earth, in which Kirk and Spock first try to foil and then work with agent Gary Seven is my favorite. The part where Roberta Lincoln (played by a young Teri Garr) is dumbfounded by the typewriter that transcribes everything she says was great. (Of course, we now take that for granted, and it's only the 21st century.)

The most riveting time travel movie I've ever seen was "Primer," which took several viewings to begin to comprehend. It was shot for only $7k and is available to rent or buy on iTunes.

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Well, I was really a young kid when I saw that Time Tunnel episode, around five or six or so, but it made a great impression on me - and I do recall my riveted fascination and my stupefied amazement that my mom and the pair of reluctant travellers all knew what was going to happen when the passengers and crew of the liner clearly didn't and remained in a star of sturdy denial.

Re Star Trek and time travelling, both TOS and STNG, I have to say that I always loved clever or well done time travelling episodes, and really liked the episodes you wrote about, as well as a number that STNG managed to do.

As someone who became an historian by profession, I love a programme which does not traduce the history but which can play with clever, witty, knowing looks at history.

I'm surprised no one has mentioned The Terminator series yet. It always bothered me that the creators/writers made such an obvious time travel mistake in that series. If my memory holds true the story says that the amazing technology of Skynet was created because of the exposure that the scientist/engineer had to parts remaining from the destroyed Terminator who was sent back to kill Sara Conner in the first place. What?!? That's not possible. Skynet can't be created in the past from it's own technology in the future. In the original timeline Skyline had to be created somehow (without this exposure) so that it could even exist in the future to begin with.

I'm surprised no one has mentioned The Terminator series yet. It always bothered me that the creators/writers made such an obvious time travel mistake in that series. If my memory holds true the story says that the amazing technology of Skynet was created because of the exposure that the scientist/engineer had to parts remaining from the destroyed Terminator who was sent back to kill Sara Conner in the first place. What?!? That's not possible. Skynet can't be created in the past from it's own technology in the future. In the original timeline Skyline had to be created somehow (without this exposure) so that it could even exist in the future to begin with.

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See, Terminator is an interesting one. I look at it like this:

Skynet Prime was created, possibly at a distant point in the future, goes rogue, bombs, work camps, human slaves, yadda yadda. Skynet Prime invents time travel and attempts to subvert John Connor Prime as the human resistance leader.

From that moment on, the entire timeline is fractured, and Skynet and John Connor exist in a loop. Every version of Skynet and every version of John Connor are now inevitably linked, but there had to have been a Prime John Connor and a Prime Skynet to start the whole process.

My favorite time loop story is Heinlein's "All You Zombies". It's...unsettling.

I'm surprised no one has mentioned The Terminator series yet. It always bothered me that the creators/writers made such an obvious time travel mistake in that series. If my memory holds true the story says that the amazing technology of Skynet was created because of the exposure that the scientist/engineer had to parts remaining from the destroyed Terminator who was sent back to kill Sara Conner in the first place. What?!? That's not possible. Skynet can't be created in the past from it's own technology in the future. In the original timeline Skyline had to be created somehow (without this exposure) so that it could even exist in the future to begin with.

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Lol, I did over look that and T1 and T2 are favorite movies. That -is- the time travel paradox, on one hand it's chicken or egg syndrome, on the other, it's self fulfilling prophecy. Maybe the idea was all ready floating around in the engineer's head, and the fragment from the first Terminator just helped him along. But how about Kyle Reese becoming John Connor's father?! As I recall somewhere in the there Sara Connor says somthing like "it's hard to wrap your head around". Personally, from a practical standpoint, I view time travel stories as cleaver science fiction, but still can enjoy them.

Yes, that film was most excellent! Strange things were indeed afoot at the Circle K. I bet Keanu Reeves cringes whenever anyone mentions that movie.

The book Replay, by Ken Grimwood, is also one of my favorites in the very broad sense of time travel. It's about a man who dies and wakes up as his 18-year-old self, but with all his memories intact. It's a great read that I've wished would be made into a movie since I first came across it about 25 years ago.

Not favorite- Terminator Genisys (2015). What results when you have too many time lines, resulting in a convoluted mess, and/or are just trying to hard to milk it, resulting in a sore teat and brain trying to keep all the variances connected and coherent.

Star Trek: The Next Generation: Yesterday's Enterprise. One of my all time favorite episodes of any tv series.

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I remember thinking how mean to kill Tasha Yar off until I realized she decided to leave. For anyone not familiar, this was a reprisal of that role. Of interest she had been originally cast as Deanna Troy, but got switched and was frustrated with the limited nature of her role as compared to other characters. All Goood Things stands as my favorite time paradox STNG episodes.

I remember thinking how mean to kill Tasha Yar off until I realized she decided to leave. For anyone not familiar, this was a reprisal of that role. Of interest she had been originally cast as Deanna Troy, but got switched and was frustrated with the limited nature of her role as compared to other characters. All Goood Things stands as my favorite time paradox STNG episodes.

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I'm a fan of pretty much all of the time travel (and non-time travel) ST episodes, and somehow Cause and Effect always comes up in my mind as one of the more interesting (although in some ways simpler) ones--not as much as time travel per se, but more of time repetition in a sense.

I'm a fan of pretty much all of the time travel ST episodes, and somehow Cause and Effect always comes up in my mind as one of the more interesting (although in some ways simpler) ones--not as much as time travel as time repetition in a sense.

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